The present invention relates to a composition for a dry lubricant film and a plain bearing with a sliding layer using the same.
Conventionally a bearing for automobile engines is fabricated by bonded a steel backing plate with a copper-based or aluminum-based bearing alloy. For such plain bearings, to improve wear resistance, anti-seizure property and initial conformability, a sliding layer is formed by coating a surface of a bearing alloy layer with a thermosetting resin, such as a polyamide-imide resin (PAI), a polyimide resin (PI) and an epoxy resin (EP), containing a solid lubricant and the like as disclosed in JP-A-4-83914 (Patent Document 1), JP-A-9-79262 (Patent Document 2), etc. Further, to improve initial wear resistance while maintaining the conformability, a protective layer is formed of a solid lubricant and a binder composed of a thermoplastic resin and a thermosetting resin soluble in a polar solvent as disclosed in JP-A-2001-343022 (Patent Document 3).
There is a known phenomenon that in using a plain bearing, cavities (bubbles) are generated in lubricating oil, which causes as a result erosion on the bearing surface. This is a phenomenon that cavities generated in lubricating oil explode under a higher pressure and energy upon exploding destructs erosively the bearing surface. A conventional countermeasure thereagainst has been to increase the material strength of a sliding layer to enhance cavitation resistance. For example, a sliding layer is constructed with a polybenzoimidazole resin (PBI) containing a solid lubricant to improve wear resistance, anti-seizure property and cavitation resistance according to JP-A-2004-19758 (Patent Document 4). Improvement of a sliding property is tried by a combination of a solid lubricant containing lead and a resin binder of at least one of PAI, PI, EP and PBI as disclosed in JP-A-2003-56566 (Patent Document 5).
Patent Document 1: JP-A-4-83914
Patent Document 2: JP-A-9-79262
Patent Document 3: JP-A-2001-343022 (paragraph 0007)
Patent Document 4: JP-A-2004-19758
Patent Document 5: JP-A-2003-56566.
The disclosures of Patent Documents 1 to 5 have not fully satisfied plain bearing performance (anti-seizure property, initial conformability and cavitation resistance) of higher level of performance and load required by recent high power, high speed internal combustion engines. For example, although Patent Document 5 describes a resin binder prepared by blending PAI, PI, EP and PBI, simple polymer-blending disperses resins in a form of clusters but does not mix the resins compatibly with each other. Consequently, it has a drawback that due to physical property nonuniformity existing in a sliding layer, sufficient anti-seizure property, especially cavitation resistance cannot be attained.
Patent Document 3 describes “a thermosetting resin and a thermoplastic resin are, when dissolved in a solvent, mixed with each other compatibly in an ultra-fine unit close to a molecule . . . come to have intermediate properties”, thereby “mixed with each other compatibly” means that a thermosetting resin and a thermoplastic resin form clusters in a solvent, although the cluster sizes are ultra-fine (microclusters). The thermosetting resin and the thermoplastic resin are therefore not mixed with each other compatibly, and the thermoplastic resin is dispersed microscopically in the thermosetting resin to exhibit intermediate properties. It still has a drawback that, when utilized as a sliding layer of a plain bearing to be used under the condition where the cavitation phenomenon should occur, the stress of cavitation concentrates at the boundary surface between the respective resin phases, where continuity of the physical properties is lost, and the cavitation resistance is deteriorated.
The present invention is conducted under the aforementioned circumstances, and an object of the prevent invention is to provide a composition for a dry lubricant film cables of further improving the bearing properties, particularly anti-seizure property, initial conformability and cavitation resistance, and a plain bearing utilizing the composition for a dry lubricant film as a sliding layer.